Effect of boron vacancies on mechanical properties of ReB2 from first-principles calculation

The lattice parameters, vacancy formation energies, elastic properties, Vickers hardness and electronic structure of ReB2 with lower concentration of boron vacancy are studied using first-principles approach. The lattice parameters and unit-cell volume of ReB2 with boron vacancy rapidly decrease as boron vacancy concentration increase. The calculated vacancy formation energies show that the ReB2 are more stable than that of system with boron vacancy. With increasing boron vacancy, the bulk modulus, shear modulus, Young modulus and Vickers hardness gradually decrease and the boron vacancy results in mechanical transition from brittleness to ductility, which are in good agreement with experimental results. The decreases of elastic modulus and Vickers hardness are originated from the weak hybridization between Re atoms and B atoms in boron vacancy region and forms the weak Re-B bonds and B-B covalent bonds.